Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF). Commercially employed fluoropolymers also include fluoroelastomers. Fluoropolymers and their applications are further described in “Modern Fluoropolymers”, edited by John Scheirs, John Wiley & Sons Ltd., 1997, or in “Fluoroplastics”, Ebnesajjad, Sina, Plastics Design Library, Norwich, N.Y., USA, 2003. Preparing such fluoropolymers generally involves the polymerization of gaseous monomers, i.e. monomers that under ambient conditions of temperature and pressure are present as a gas. Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization (disclosed in e.g. U.S. Pat. No. 3,855,191, U.S. Pat. No. 4,439,385 and EP 649 863), aqueous emulsion polymerization (disclosed in e.g. U.S. Pat. No. 3,635,926 and U.S. Pat. No. 4,262,101), solution polymerization (disclosed in e.g. in U.S. Pat. No. 3,642,742, U.S. Pat. No. 4,588,796 and U.S. Pat. No. 5,663,255); polymerization using supercritical liquids (disclosed in e.g. JP 46011031 and EP 964 009) and polymerization in the gas phase (disclosed in e.g. U.S. Pat. No. 4,861,845).
Currently, the most commonly employed polymerization methods for fluoropolymers include suspension polymerization and especially aqueous emulsion polymerization. In aqueous emulsion polymerizations the polymerization is carried out in the presence of a fluorinated emulsifier, which is generally used to accelerate the reaction rate and to stabilize the polymer particles formed.
However, fluorinated emulsifiers are expensive materials and attempts have been made to reduce their use. Certain non-emulsifying fluorinated compounds, so-called ‘doping agents’, have been added to the reaction mixture which also increase the reaction rate allowing for reducing the amounts of emulsifiers. However, the presence of doping agents also leads to a reduction of the particle sizes of the fluoropolymers formed as is shown in EP 1 245 596. For example, the particles of fluoropolymers at a given solid content are smaller when doping agents are used compared to the same reaction carried out without doping agents. For example, in U.S. Pat. No. 5,895,799, aliphatic or aromatic perfluorinated hydrocarbon used as doping agents have been reported to lead to fluoropolymers having particle sizes of 80 nm or less when used in a weight ratio with respect to the emulsifier of approximately 1:1. Doping agents in the form of perfluoropolyethers have been reported to lead to particles of less than about 70 nm (EP 250 767).
The generation of small fluoropolymer particles may be desirable in many applications and due to their known effect of reducing the particles sizes doping agents have been employed for this purpose.
However, in some applications, the formation of larger rather than smaller fluoropolymer particles is desired. Such applications include, for example, those where thick fluoropolymer coatings are required. Thick coatings can be prepared with fewer coating steps using fluoropolymers with large particle sizes. In other applications bimodal particle size distributions, e.g. a fluoropolymer composition where one polymer population has small particles sizes and another polymer population has large particle sizes may be desirable, because such dispersions can lead to particularly dense coatings.
Therefore it would be desirable to provide a process for the preparation of fluoropolymers by aqueous emulsion polymerization using doping agents by which at a given reaction time or solid content larger polymer particles can be formed. It is further desirable that the process can be practiced with commonly employed fluorinated emulsifiers.